Currently, electric energy-storage devices having large capacity are required in the fields such as midnight-power storage and auxiliary power supplies for power failure. Electric energy-storage devices having large capacity per unit volume, that is, having abilities of supplying power for long duration in spite of the small size thereof, are also required in the field of transportation vehicles characterized by battery-powered driving force such as battery-powered electric vehicles and hybrid electric vehicles and in the field of portable electric terminals such as mobile personal computers, cellular phones and portable audio devices.
Electric double-layer capacitors are expected as an electric energy-storage device, which are composed of an electrode, a separator and an electrolytic solution and store electric power in a boundary surface (electric double-layer) formed between an electrolyte and an electrode due to absorption of the electrolyte dissolved in an the electrolytic solution to the electrode. The capacity of stored energy is defined by the formula ½·C·V2 (wherein C is electrostatic capacity (F) and V is voltage), therefore in order to store more energy, the electrostatic capacity of electric energy-storage devices must be enhanced, especially in order to store more energy in compact volume, enhancement of electrostatic capacity per unit volume is required.
Activated carbons are commonly used for the electrode of electric double-layer capacitors, specifically included is an activated carbon having micropores (20 Å or less of pore diameter) as major pores thereof which is obtained by carbonizing and activating palm shell and the like.
Recently, an activated carbon having meso-pores as major pores thereof is disclosed which is produced by polymerizing resorcinol with formaldehyde in the presence of a basic catalyst and an aqueous solvent to obtain an organic aerogel uniformly having meso-pores (20 Å or more of pore diameter), washing the organic aerogel with an organic solvent, replacing the aqueous solvent to the organic solvent, drying and then carbonizing, and can be used for the electrode of electric double-layer capacitor (U.S. Pat. No. 4,873,218), however, the electric double-layer capacitor does not have sufficient electrostatic capacity per unit weight.
It is also reported in Chemistry Express, Vol. 5, No. 8, pp. 606-608 (1990) (KAZUHISA MURATA, TAKASI MASUDA, AND HISASI UEDA) that a cyclic tetramer which is a polymer of resorcinol and an aldehyde compound can be carbonized, however the electrostatic capacity given by the carbonized material is not sufficient.
The inventors of the invention have studied about activated carbons capable of giving electric double-layer capacitors having large electrostatic capacity, and found that an electric double-layer capacitor having an electrode which contains an activated carbon obtained by carbonizing a cyclic polymer of resorcinols and an aldehyde compound can exhibit a sufficient electrostatic capacity as expected.